In conventional helicopters, the rotor blade pitch is varied both collectively and monocyclically. The rotor blade pitch is varied collectively to control vertical lift or altitude. To make the helicopter travel in directions other than vertical, the rotor blades are tilted monocyclically.
In controlling vertical lift or mean thrust, the collective pitch is set without regard to the rotational position of each blade. The change in lift on most helicopters is created by a change in blade pitch which alters the blade's angle of attack relative to the air. The higher the angle of attack, the more highly loaded the blade becomes in creating more lift. The helicopter's engine must produce greater power to maintain the generally constant speed of rotation when blade loading is increased.
Alternatives to pitch control of helicopter lift which have been applied include blade flaps, jet flaps and circulation control. A blade flap comprises a hinged partial span of the blade used to deflect air for greater or lesser lift. Jet flaps and circulation control are more subtle methods of changing lift that rely on changes in generated air currents rather than on mechanical reconfiguration. Furthermore, it is also possible to adjust lift by changing the blade rotation speed but this is not commonly done.
To change the direction of thrust longitudinally or laterally from the vertical, the pitch of each blade is varied by a given amount once per rotor revolution, the blades being out of phase with each other. Generally the rotor of a conventional helicopter is made to tilt by having one half of the disc traced out by the rotating blades achieve more lift than the other half. Normally this results in the pitch of a blade going through a complete cycle from maximum to minimum during one revolution. Cyclic pitch control as just described can also be supplanted by the previously described alternate lift control methods if they are applied monocyclically.
The pitch of each blade in a conventional rotor is controlled by a control rod, and the positions of all such rods are controlled by a single swashplate. The control rods are mounted circumferentially around the swash-plate so that axial movement of the plate causes collective changes in pitch. Longitudinal and lateral tilting of the swash-plate results in cyclic pitch control. The pitch of each blade may be set directly by the control rod or through mechanical flaps on the blades or the like.
FIG. 1 illustrates a typical swash-plate system. A plate 12 is mounted for rotation with the rotor 14 about the rotor axis. The axial position of plate 12 along the rotor shaft and the tilt of the plate are determined by a nonrotating plate 16 which is in turn moved by a number of plate actuating rods 18. The pitch of each rotor blade 20 at any angle of rotation is controlled by the axial position of a rod 22. If the plate 12 is level, all rods 22 hold the several blades at the same constant pitch. When, however, the plate 12 is tilted by selective actuation of plate actuators 18, each rod 22 reciprocates one cycle for each revolution of rotor 14 and plate 12. The result is a monocyclic pitch change.
The rotor blades operate in a severe aerodynamic environment. Flapping and bending of each blade can be caused by atmospheric turbulence, the vortices of preceding blades, fuselage interference, and aerodynamic or mass mismatch between blades. Flapping and bending might also result from the changing velocity of the blade relative to the surrounding air with rotation; that change in velocity can result in retreating-blade stall flutter during flight or sailing during shutdown.
The most significant modes of undesired blade motion follow. In the blade flapping mode for a hinged blade, shown in the solid lines of FIG. 2A, a blade 24 flaps on a hinge 26. For a hingeless blade, the corresponding mode is the first flatwise bending mode of the blade. The first flatwise bending mode of a hinged blade is shown in broken lines in FIG. 2A, and such bending corresponds to the second flatwise bending mode of a hingeless blade. FIG. 2B illustrates the first inplane bending mode of a blade where there is no inplane hinge near the root of the blade. Inplane bending or flapping is referred to as lagging. Finally, in a first torsion mode, the blade is subjected to a twisting action along its own axis. The flapping mode (first flatwise bending mode for hingeless rotors) and the first inplane bending mode are excited by gusts and other natural air turbulence and by blade instabilities; they are generally of relatively low frequencies, that is of frequencies less than the rotor frequency. On the other hand, the first flatwise bending mode (second flatwise mode for hingeless rotors), and the torsion mode are excited by vibration and stall flutter which are relatively high frequency disturbances.
To reduce the instability of the rotor with the above environmental conditions and to reduce the resultant stresses and power losses, Marcel Kretz has proposed the use of individual actuators for the rotor blades, each actuator being controlled by a feedback circuit. Vertica, 1976, Vol. 1, pp 95-105. Feedback is provided by pressure sensors mounted on each blade or by strain gauges or the like associated with the blade. The resultant control system provides for multicyclic individual blade control which reduces the adverse effects of the environment while still providing for the usual collective and monocyclic controls by the helicopter pilot.
An object of the present invention is to provide individual control of the rotor blades with feedback of higher reliability and having better control characteristics than has heretofore been available.